To illustrate various problems which arise in the evaluation of accident counter-measures this paper presents a case study of the public road trials of a design of speed control hump developed at TRRL. The objectives of the study are outlined together with details of the legal and practical constraints affecting its design and the various evaluation measures used. The effectiveness and acceptability of humps as a means of improving safety and amenity on residential roads was investigated. The period for which the experimental humps could remain in position was restricted by law to one year which thus also restricted the length of evaluation period. The way in which suitable sites were selected for study is discussed. The study took various measures of the effect of the humps on vehicle speeds on the roads concerned (i.e. a 'process' evaluation), on the previously untested assumption that a reduction in speed would reduce accidents on such roads. There was also a resultant indirect effect on accidents via changes in vehicle flows. The ultimate evaluation, made in terms of accident changes (the 'product' evaluation) is also discussed. A survey of public opinion at the experimental sites was carried out and the importance of this (as a possible further 'process') is considered. The paper also outlines the problems associated with selecting appropriate control data. Both types of evaluation techniques demonstrate the need for area-wide studies and the value of monitoring as many variables as possible to enable all the advantages and disadvantages of the measure to be assessed. (a) for the covering abstract of the conference see IRRD abstract no 264967.
Abstract